How does your submission match the theme?
It's explained on the project page, but:

Countdown: Forces players to fight instead of camping, and makes sure rounds don't last too long and stay fun.

Shooting: The primary tool to kill the other player, but also useful for destroying certain walls and activating traps.

Ammo packs: The players have only 2 bullets. If you run out you can grab an ammo pack to get your bullets back. Also forces players into conflict, because players will be drawn towards the ammo pack.

Breakable walls: Make it harder for the players to reach and kill eachother in the time period, and they waste bullets.

Explosive thingamajigs: Can kill players if they're caught up in the explosion, but its main purpose is to waste bullets.

Lazers: Block your path, and are able to kill you.

Third-party resources
GameMaker 1.4
GIMP
SFX provided by FreeSound
Music provided by Bensound

Contributors
Developer & Tester: Me
External contributors: Freesound & Bensound